Atomic manganese {sup 55}Mn and chromium {sup 52}Cr are simultaneously loaded and confined in a magnetic trap. Using a cryogenic {sup 3}He buffer gas, 10{sup 11} manganese and 10{sup 12} chromium atoms are trapped at an initial temperature of 600 mK. The buffer gas is then pumped away, thermally isolating the sample. The Mn-Cr interspecies inelastic rate constant is measured to be {gamma}{sub Mn,Cr}=1.5({+-}0.2)x10{sup -13} cm{sup 3}/s.